I owe someone on Reddit an apology. During my last run of the mod, I tried to not use Diminish at all but I deemed it necessary for this fight because of Iceberg spam. I tested out my claim in this run and found myself very wrong.

The Giant Gloop seems to trigger Iceberg when you repeatedly attack it with Red Elements. That is the best I can tell after a lot of testing. What I can tell you for certain is this is the only way I won, throwing in more of a Yellow offense even if it did much less damage. I picked Yellow because of my team but I imagine anything besides Blue will work just as fine.

GRECO:
Iron Glove
Iron Mail
Power Glove
Dragoon's Honor
Sight Scope

MEL:
Steelerang
Iron Vest
Moonglasses
Sky Djinn Ring
Bronze Helmet

PIP(Yellow Devil Form)
Iron Glove
Iron Mail
Moonglasses
Magic Ring
Bronze Helmet